The NC machining technique is used in forming a machining plane in a material by relatively moving a tool and/or the material along a predetermined movement path. The ‘machining plane’ herein refers to an actual surface plane to which the material is to be formed as a result of the processing by the tool. In the NC machining technique, the shape of the machining plane is represented by a machining plane model calculated using a three dimensional CAD (Computer Aided Design) apparatus. In order to form a desired machining plane in the material (,) in the course of actually performing tool processing using the NC machine tool, the movement path of the tool should be predeterminedly calculated based on the machining plane model that represents the desired machining plane.
In the NC machining technique, a tool reference point is determined in the tool in order to represent the position of the tool using coordinates. The movement path of the tool can be determined based on the movement path of the tool reference point. The ‘tool reference point’ herein refers to a point that can voluntarily be determined within a tool shape. For example, in a case where a tool reference point and a knife edge of a tool are respectively determined, when the knife edge of the tool is moved while the edge being in contact with the machining plane of the work material, the movement path of the tool equals the path along which the tool reference point moves. As a result, the movement path of the tool is positioned on a plane that is offset from the machining plane by an amount equal to the tool shape. In order to determine the movement path of the tool, it is necessary to calculate a tool reference plane model that is offset from the machining plane by an amount equal to the tool shape based on the machining plane model that represents the machining plane. The ‘tool reference plane model’ is a plane created from a sequence of a movement of the tool reference point.
Japanese patent application publication No. 06-25744 teaches a technique for calculating a tool reference plane model utilizing an inverse offset method. The inverse offset method utilizes an inversed tool model system in which a machining plane model and the tool shape are inversed with respect to the position of the machining plane model and the tool shape in the actual machine processing. The method calculates an enveloping plane model of the inversed tool models having its tool reference point positioned over the machining plane model. The method is able to calculate a tool reference plane model that is offset from the machining plane model by an amount equal to the tool shape.